(1) Field of the Invention
This invention relates to key switch devices, and particularly, to key switch devices wherein a key is depressable to allow a movable contact to come into contact with a pair of fixed contacts.
(2) Description of the Prior Art
A conventional key switch device, which is used in various electrical and electronic devices, for example, a key telephone set, includes an electric circuit board, an insulator housing plate fixedly disposed on an upper surface thereof and having an opening, and a key slidably fitted in the opening and elastically supported therein. The key is accompanied with a movable contact which moves together with the key. The circuit board has a pair of fixed contacts formed on the upper surface. The pair of fixed contacts is closed or connected with one another by the movable contact when the key is pushed down against the elastic support so that the movable contact comes into contact with the fixed contacts.
It has been known that such a key is provided with elastically flexible leg portions extending downwardly, each of which has engaging means for engaging with the housing plate. When the key is mounted on the housing plate, leg portions are slightly bent inwardly in a radial direction so as to be inserted in the opening of the housing plate. Therefore, it is desired that each of the leg portions can be readily elastically bent. After that, the key is straightly guided by the inner peripheral surface of the opening of the housing plate and that leg portions are restored so that engaging means engage with the opening edge of the housing plate, to thereby prevent the key from slipping out of the opening of the housing plate. In this construction, since the key can be inserted into the opening from the upper side of the housing plate which has been fixedly secured to the circuit board, the key switch device can be readily assembled.
Particularly, if the leg portions are longer, the mounting of the key on the housing plate can be readily carried out because the leg portions can be readily bent inwardly. However, if a force is applied to the key at a position offset from the center of the key to push down the key, the key is inclined from the center axis of the opening because the leg portions are elastically bent inwardly. As a result, the key is not smoothly and straightly moved along the center axis of the opening of the housing plate. Therefore, the movable contact does not reliably come into contact with the pair of fixed contacts.
On the other hand, if the leg portions are shorter, it can be difficult to mount the key on the housing plate. Because the leg portions can not be readily bent inwardly, the engaging means engages with the edge of the opening of the housing plate thereby to prevent the leg portions from being inserted into the opening.
In order to insure a reduced inclination of the key pushed at its peripheral edge, the leg portions are provided to a peripheral edge of the key in the known key switch device. In the case, use of a key having an increased area of the top surface means that the opening of the housing plate is large, which results in a large key switch.